I was thinking the other day, and came up with a couple of mission mechanics that could be added after launch, but would probably be best if planned for ahead of time.
The first one is taken from Warframe. Basically the mission would be divided up into zones and in each zone there are a number of alarm panels. When the mobs see you there is a random chance that one of them would run to set off the alarm. If the alarm is triggered then more mobs are sent against you in that zone. I don't really think it should be endless waves, but may just take the number of mobs in the zone and double it, sending the second batch as runners released over a 5 minute period. You could stop the addons from being released simply by disarming the alarm, which in warframe is a minigame. I'm not sure that part is necessary or not, but it is a possibly. Of course if you prevented the runner from ever reaching the alarm, then no extra mobs would be released, although I think having the ability to destroy the alarm before anyone gets to it should also be a possibility, too. Also in Warframe, if the alarm is triggered doors drop down and lock you into the area until the alarm is disarmed, which leads me to the second mechanic.
The second suggestion I have is to have the ability to blow open doors in a mission regardless of whether they were created from an alarm or placed there during mission design. How the door was blown open would be based off of the offensive power set a character has. I basically see it as a hidden passive level one power tied to each offensive power set. What it looked like would be defined by what animation set or props a character has at their disposal. So how it would work is that when you interacted with the door it would ask you how you wanted to open the door. Do you want to slam it with your shoulder, hit it with your sledge hammer, or let the hamster riding on your shoulder "Go for the lock, Boo"? Once you had selected the appropriate animation it would be a 10 sec charge power. If you charge it half way, it would blow open the door in whatever fashion you chose. If you charge it longer than that, then when it blows open the door any mobs within, let's say, 15 ft receive a scaling stun specific to the power set and take minor damage, so that at a full charge they would receive a 3 second stun plus oh I don't know how about loosing 3% of their health. Of course all the numbers would have to be worked out, but you get the idea. Also, I was thinking it could be kind of interesting to make it so that there is a slight difference between a melee door opener and a ranged. So for a melee it might have a 4 second stun and a ranged might do 4% damage. The other option to open the door would be going and finding the key which could be on a mob in a specific location, a random mob that area, or a wandering mob you would have to hunt down. The advantage of using the key would be that if you blow open the door, you automatically set off the alarm in the zone behind the door, but if you use the key then you don't set off the alarm.
Some possible power set door openers could be:
- Burning: When the door opened it would explode into chunks of molten/burning shrapnel propelled into the next room setting the mobs on fire, who would stand around trying to put them out for the appropriate time period.
- Ice: When the door opened it would explode into ice shards doing damage to the mobs behind the door and causing them to be frozen for a few seconds.
- Lightning: The door would just swing open, but when it did you would see the mobs being electrocuted on the other side.
- Smashing: The door would fly 5 to 10ft into the next room causing a concussion blast that would knock them back causing them to take damage and stunning them
- Ranged Burning: The character projects a stream of fire at the center door which causes it to start to glow the color of the fire stream. This expands until at full charge it has a diameter from floor to ceiling. Once the power is either fully charged or released, the stream stops and the character throws a fireball which cause the door to explode.
- Burning Melee: The character puts his hands on the door and the spot under them would start to glow the appropriate color until it, too, is floor to ceiling at a full charge. At the release or full charge the character punches it and it explodes.
- Burning Melee with hammer prop: The character hammers on the door leaving round glowing circles every place the hammer hits. At full charge the circles will have covered ninety percent of the door and then the character brings the hammer back for a two handed over the head swing which causes the door to explode.
- Ranged Ice: The character would project a stream of ice causing the door to be covered in an ever increasing thickness of ice. At release or full charge the character would throw a little snow ball at it which would cause it to explode into ice shards.
- Smashing Boxing: The character does a little shadow boxing before doing a roundhouse punch to knock the doors into the next room.
- Smashing Brawling: The character pushes against the door with his shoulder until he is 49% charged, at which point he starts backing up. When it is released or fully charged he runs forward ramming the door with his shoulder causing it to fly into the other room.
- Lethal Katana: For the first 50% the character stands in front of the door studying it. If the power is release at this point, he would do an over head strike splitting the door in half. If he continues to charge it he starts slashing the door until he finishes with the over head strike which would cause the door to fly into the other room in 8 to 10 pieces.
Anyways those are my ideas.